WASHINGTON - Presidential politics took a back seat to tragedy Friday as Americans grappled with yet another mass shooting, this one an estimated 90-second rampage in a Colorado movie theatre that left 12 people dead and 59 injured.

"This morning we woke up to news of a tragedy that reminds us of all the ways we are united as one American family," President Barack Obama said in brief remarks in Fort Myers, Fla., where he'd been scheduled to make a campaign speech aimed at rallying supporters.

Instead, he cut short his swing through the key battleground state and headed back to Washington, D.C.

"There are going to be other days for politics," he said. "This, I think, is a day for prayer and reflection."

The carnage occurred in a crowded movie theatre in suburban Denver when a gunman opened fire as movie-goers watched a midnight screening of "The Dark Knight Rises."

One of the victims, Jessica Ghawi, was an American hockey writer who survived Toronto's recent Eaton Centre shooting.

Obama also vowed that the perpetrator of the "heinous act" would be brought to justice. Authorities have arrested a lone gunman identified as James Holmes, 24, a neuroscience PhD student.

"Such violence, such evil is senseless. It's beyond reason," the president said before ordering American flags to be flown at half-staff until sunset Wednesday.

Mitt Romney, Obama's Republican rival for the White House, released a statement saying that he and his wife, Ann, were "deeply saddened by the news of the senseless violence."

In an appearance in New Hampshire, he added: "Our hearts break with the sadness of this unspeakable tragedy."

Both men suspended campaigning and also stopped airing negative ads in the state of Colorado.

The bloodshed in Colorado is just the latest mass shooting to play out in the United States, a nation that increasingly reveres its guns no matter how many times such incidents occur.

Even the near-fatal shooting of one-time Democratic congresswoman Gabrielle Giffords in a similar incident in January 2011 has failed to result in tougher gun control laws in the U.S.

Indeed, Gallup polls suggest Americans have grown more resistant to tougher gun-control laws over the past two decades. Many states have loosened their gun laws, allowing citizens to carry loaded firearms in public as the National Rifle Association continues to wield formidable clout on Capitol Hill.

In Colorado, in fact, people are allowed to have handguns in their cars, businesses and homes and can carry concealed weapons in public.

New York Mayor Michael Bloomberg, a longtime gun control advocate, assailed both Obama and Romney in a sharp rebuke on Friday morning.

"You know, soothing words are nice, but maybe it's time that the two people who want to be president of the United States stand up and tell us what they are going to do about it," Bloomberg said in an interview with a New York radio station.

"There are so many murders with guns every day — it's just got to stop.

"And instead of the two people — President Obama and Gov. Romney — talking in broad things about they want to make the world a better place. OK, tell us how.... No matter where you stand on the Second Amendment, no matter where you stand on guns, we have a right to hear from both of them concretely, not just in generalities. Specifically, what are they going to do about guns?"

Jay Carney, the White House press secretary, told reporters earlier Friday that Obama is well aware of the need to balance upholding the Second Amendment — which enshrines the right to bear arms in the U.S. Constitution — with shielding citizens.

"The president believes we need to take common-sense measures that protect the Second Amendment rights of Americans while ensuring that those who should not have guns under existing laws do not get them," Carney said.

Those opposed to gun control, meantime, fume any time it's brought up in the aftermath of a mass shooting. Friday was no exception.

"If you use the Aurora tragedy to promote gun control, you are despicable," read a typical headline Friday on RedState.com, a conservative website.

As always, the emotionally charged and seamy underside of American politics revealed itself on Friday, with the Drudge Report posting an item suggesting Holmes was a registered Democrat while an ABC reporter suggested he was a Tea Party adherent.

The network later apologized for the suggestion.

A Republican congressman from Texas, meanwhile, linked the shooting to the "ongoing attacks on Judeo-Christian beliefs" while also wondering aloud why no one in the packed theatre was able to return the gunman's fire.

"What really gets me, as a Christian, is to see the ongoing attacks on Judeo-Christian beliefs, and then some senseless crazy act of a derelict takes place," Louie Gohmert said in an interview with the Heritage Foundation.

"We've threatened high school graduation participations ... if they use God's name, they're going to be jailed," he said. "Where was God? What have we done with God? We don't want him around. I kind of like his protective hand being present."

Gohmert went on to ask why no one — neither God nor mere mortal — was able to halt the gunman in the midst of his murderous rampage.

"With all those people in the theatre, was there nobody that was carrying a gun that could have stopped this guy more quickly?"

Gohmert is one of five Republicans already in hot water this week for alleging an aide to Secretary of State Hillary Clinton, Huma Abedin, is a spy for the Muslim Brotherhood.